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An ancient pathway combining carbon dioxide fixation with the generation and utilization of a sodium ion gradient for ATP synthesis - PubMed

An ancient pathway combining carbon dioxide fixation with the generation and utilization of a sodium ion gradient for ATP synthesis

Anja Poehlein et al. PLoS One. 2012.

Abstract

Synthesis of acetate from carbon dioxide and molecular hydrogen is considered to be the first carbon assimilation pathway on earth. It combines carbon dioxide fixation into acetyl-CoA with the production of ATP via an energized cell membrane. How the pathway is coupled with the net synthesis of ATP has been an enigma. The anaerobic, acetogenic bacterium Acetobacterium woodii uses an ancient version of this pathway without cytochromes and quinones. It generates a sodium ion potential across the cell membrane by the sodium-motive ferredoxin:NAD oxidoreductase (Rnf). The genome sequence of A. woodii solves the enigma: it uncovers Rnf as the only ion-motive enzyme coupled to the pathway and unravels a metabolism designed to produce reduced ferredoxin and overcome energetic barriers by virtue of electron-bifurcating, soluble enzymes.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. The enzymes of the Wood-Ljungdahl pathway (a) and the corresponding gene cluster (b).

Cluster I: two genes for formate dehydrogenase (fdhF) are organized together with three copies of a FeS-containing subunit of a [FeFe]-hydrogenase (hycB), a formate dehydrogenase accessory protein (fdhD) and another subunit of the [FeFe]-hydrogenase (hydA2). Cluster II: genes for formyl-THF synthetase (fhs1), methenyl-THF cyclohydrolase (fchA), methylene-THF dehydrogenase (folD) and methylene-THF reductase (metF, metV) are organized together with a RnfC-similar protein (rnfC2). Cluster III: genes for the subunits of the CO dehydrogenase/acetyl CoA synthase complex consisting of CO dehydrogenase (acsA), acetyl-CoA synthase (acsB1), corrinoid-iron sulfur protein (acsCD) and methyltransferase (acsE) are organized together with two copies of a CODH nickel-insertion accessory protein (cooC), a corrinoid activation/regeneration protein (acsV) and two hypothetical proteins (Orf1 and Orf2).

Figure 2
Figure 2. Genetic organization and subunit composition of the [FeFe]-hydrogenase of A. woodii.

Awo_c27000 (HydE) is not a component of the active enzyme but may be involved in assembly or signalling.

Figure 3
Figure 3. Subunit composition of the formate dehydrogenase of A. woodii.

fdhF1 and fdhF2 code for sulfur and selenium containing isoenzymes, respectively. HycB1 and HycB2 may be specific for FdhF1 and FdhF2, respectively. The electron transfer subunits HycB1, HycB2 and HycB3 each have four conserved tertacysteine motifs.

Figure 4
Figure 4. Subunit composition and model of the methylene-THF reductase of A. woodii.

Panel A depicts an indirect coupling via NADH+H+ and the small subunit of the methylene-THF reductase (MetV) as electron input module, panel B a direct coupling of the methylene-THF reductase to the Rnf complex via RnfC2.

Figure 5
Figure 5. A quantitative bioenergetic model for acetogenesis from H2+CO2.

The amount of ions translocated by the Rnf complex and the Na+ F1FO ATP synthase are not exactly known. For sake of clarity, a Na+/e− stochiometry of 1∶1 is assumed (based on a ΔGO′ value of −37 kJ/mol; E0′ Fd2−/Fdox = −500 mV; E0′ NADH+H+/NAD+ = −320 mV; and ΔuNa+ (electrochemical Na+ potential across the cytoplasmic membrane) = −320 mV). For the ATP synthase, a Na+/ATP stoichiometry of 4 is assumed. The redox potentials (E0′) are: CO2/formate = −430 mV, methenyl-THF/methylene-THF = −300 mV, methylene-THF/methyl-THF = −200 mV, CO2/CO = −520 mV.

Figure 6
Figure 6. The hydrogen threshold concentration for acetogenesis from H2+CO2.

Time course of hydrogen oxidation by cell supensions of A. woodii with CO2 as terminal electron acceptor. Assays received 1.5 (▴) or 1 ml (•) hydrogen. One assay did not contain cells (▪). At the indicated time point, hydrogen was added again to proof the viability of the cells.

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